Gasoline direct injection (GDI) engines for automotive and medium-duty applications offer greater power output and higher fuel economy compared to conventional multi-port fuel injection engines. However, GDI engines would produce carbon soot as a component of their exhaust emissions. Although the mass of soot is generally lower than that produced by diesel engines, the particle size of the soot in GDI engine exhaust would be much finer. Because a given mass of fine particles contains many more particles than the same mass of coarse particles, it is likely that an exhaust after-treatment filtration device will be desired for vehicles that are powered by GDI engines.